Printed wiring boards are formed from a layer of conductive material (commonly, copper or copper plated with solder or gold) carried on a layer of insulating material (commonly glass-fiber-reinforced epoxy resin). A printed wiring board having two conductive surfaces positioned on opposite sides of a single insulating layer is known as a “double-sided circuit board.” To accommodate even more circuits on a single board, several copper layers are sandwiched between boards or other layers of insulating material to produce a multi-layer wiring board.
The copper or other conductive material can be plated, clad or otherwise deposited onto the insulating board and then etched into the desired formations. During manufacturing, nickel can be deposited onto the copper or other conductive substrate. This can be done, for example, using the electroless nickel part of an electroless nickel/immersion gold (ENIG) technique. The process results in a layer of electroless nickel being deposited on the conductive substrate.
Depositing an electroless nickel finish onto the conductive substrate has a number of advantages. It prevents copper oxidation, facilitates solderability and prevents defects during the assembly process.
Despite these many benefits, sometimes it would be desirable to selectively remove the deposited electroless nickel from the conductive substrate. For example, in order to re-work the copper on a faulty wiring board it would first be desirable to remove the deposited nickel. It would be important that no significant amount of copper be removed during the deposited nickel removal. Ideally the copper should remain bright, uniform and un-etched. This would allow for the best re-working surface.
Previously, the deposited electroless nickel could not be selectively removed from faulty boards. Faulty boards had to be thrown away because it was impossible to remove the nickel without also removing or damaging the copper. Reprocessing of faulty boards was impossible.
Attempts have been made using chemicals to selectively remove deposited nickel while leaving the substrate unaffected. Various degrees of success have been obtained.
U.S. Pat. No. 4,554,049 (Bastenbeck) describes a solution stated to be useful for stripping electrolytic nickel and low phosphorous electroless nickel from a ferrous metal substrate that uses sulfamate ions, nitrate ions, chloride ions and an iron complexing agent. However, this solution is not intended for the removal of electroless nickel from copper on printed wiring boards. It is more difficult to selectively remove nickel from copper than from a ferrous metal. The printed wiring board industry requires a high degree of precision when selectively removing nickel from copper. This method is not believed to achieve the required precision.
U.S. Pat. No. 4,720,332 (Coffey) discloses a nickel stripping bath that is stated to utilize soluble nitrobenzene compounds, zwitterions (as chelating agents), sulfide producing compounds, carbonates and a reverse current to remove the nickel deposit. This method is believed to microscopically etch in high current density areas, which dulls the substrate surface. This method also requires an electrical contact with the substrate. A panel that has already been selectively etched is not contactable with electric current because the etching creates gaps in the conductive material (for example copper). These gaps prevent electric current from flowing to the nickel coated surfaces and stripping the nickel. This process would also etch copper along with the nickel.
U.S. Pat. No. 6,323,169 (Abe) describes a composition said to be useful as a resist stripping agent containing an oxidizing agent, a chelating agent (selected from the group of aminopolycarboxylic acids, the salts of aminopolycarboxylic acids, phosphonic chelating agents, condensed phosphoric acids and salts of condensed phosphoric acids) and a water soluble fluorine compound. However, this is a resist stripping method and is not disclosed to selectively strip nickel from copper.
U.S. Pat. No. 6,332,970 (Coffey) describes an electrolytic method said to be useful for stripping electroless nickel from iron, cast iron, steel alloy, stainless steel, aluminum, electroplated nickel, cast nickel, iron/nickel/cobalt alloy, sulfamate nickel or titanium. Coffey discloses immersing the substrate into an electrolytic bath containing an oxoacid, oxoacid salt or a combination thereof and hydrogen peroxide. As with Bastenbeck, this method is not directed to removing electroless nickel from a copper substrate for use in printed wiring boards. Furthermore, an electrolytic method may not be suitable to remove electroless nickel from a printed wiring board. As previously stated, a panel that has already been selectively etched is not contactable with electric current because the etching creates gaps in the conductive material (for example copper). These gaps prevent electric current from flowing to the nickel coated surfaces and stripping the nickel. This process would also etch copper along with the nickel.
U.S. Pat. No. 6,642,199 (Humphreys) describes a nickel stripping composition containing an oxidizing agent, an amine and an organic or boric acid. The composition in Humphreys is said to require a pH of greater than 6.5 in order for stripping to take place. A highly alkaline formula such as the one described in the '199 patent can ruin the soldermask on a printed wiring board.
U.S. Pat. No. 6,797,682 (Hara) describes a resist stripper made up of hydrogen peroxide, a quaternary ammonium salt, water and a water soluble organic solvent. However, this is understood to be a resist stripping method and is not disclosed to selectively strip nickel from copper. Furthermore, this method is not effective on printed wiring boards because the highly alkaline formula can ruin the soldermask.
Accordingly, it would be desirable to provide a method and composition for selectively stripping nickel from a printed wiring board without removing a significant amount of the underlying substrate. It also would be desirable to provide a method of stripping nickel such that the underlying substrate is bright, uniform, not significantly etched and fit for re-working after the nickel is stripped.